1. Field of the Invention
The present invention relates to a permanent magnet type three-phase stepping motor and, more particularly, to a permanent magnet type three-phase stepping motor for use in an apparatus for an office automation or the like.
2. Description of the Prior Art
The vibration generated in the permanent magnet type three-phase stepping motor is smaller than the vibration generated in the permanent magnet type two-phase stepping motor. The number of transistors required for the driving circuit of the three-phase stepping motor is six, whereas the number of transistors required for the driving circuit of the two-phase stepping motor is eight. Accordingly, the three-phase stepping motor is excellent in cost performance. Further, the precision of the positioning of the rotor and the fluctuation of rotation at the low speed can be enhanced if the pole pair number of the rotor is increased. However, the vibration and the noise are generated even in the three-phase stepping motor, because many harmonic waves are generated in the magnetic flux field made by the permanent magnet under the affection of a number of pole teeth.
It is considered that the number of the stator main poles in the three-phase stepping motor is a multiple of three, such as 3, 6, 9, 12, . . . . The motor having three stator main poles is the most simple in construction. Further, it is noted that a lesser number of stator main poles causes larger torque, because the interlinkage magnetic flux per one stator main pole becomes large. Japanese Patent No. 3140814 and the corresponding U.S. Pat. No. 5,289,064 are known as the prior art.
If such conventional stepping motor having three stator main poles is driven by the two-phase exciting manner, an unbalanced electromagnetic force is generated in the radial direction, and harmonic waves are generated due to the large number of small rotor teeth, so that the cogging torque is increased. Further, the center of the air gap between the rotor and the stator is deviated due to the unbalanced electromagnetic force in the radial direction, so that the vibration and the cogging torque are also increased.
In the stepping motor having twelve stator main poles, punched silicon steel plates can be piled while changing in angular position by 90 degrees so as to form the stator core, so that the differences in property of the silicon steel plates due to the rolling direction thereof are cancelled with one another so as to improve the magnetic balance, that the harmonic waves can be reduced because the density of the magnetic flux in the air gap becomes substantially in the form of sine wave, that the fluctuation in thickness of the stator core becomes small, and that the air gap becomes uniform. However, in case that the number of the stator main poles is six or nine, the punched silicon steel plates cannot be piled while changing in angular position by 90 degrees, so that the cogging torque becomes larger than that in case that the number of the stator main poles is twelve, because an oscillation torque is added to the torque generated according to the Fleming's Left-Hand Rule, that is, IBL rule ( where I is current, B is magnetic flux and L is thickness of piled plates ), when the windings are excited by the lower order, such as third or fourth harmonic wave.
Further, in the stepping motor of the conventional construction, if a number Nr of the small rotor teeth becomes large a tooth phase ( mechanical angle ) between the teeth of N pole and S pole of the rotor is 180/Nr and becomes small. Accordingly, the error in the electrical angle becomes large even if the error in the mechanical angle is small. Further more, in case that the number of the stator main poles is three or six, the punched silicon steel plates cannot be piled while changing in angular position by 90 degrees, so that the harmonic wave of relatively low order generated due to the unbalance of the magnetic resistance in the magnetic circuit or the unevenness of the air gap is not disappeared between the phases.
Thus, the manufactured stepping motors become different from one another in cogging torque, vibration or noise.